There are two broad classes of batteries: liquid state batteries, i.e., those with a liquid or wet electrolyte, and solid state batteries, i.e., those with an electrolyte in a solid state. Solid state batteries have advantages over liquid state batteries in that they inherently do not dry out and do not leak. However, they suffer major disadvantages when compared with liquid state batteries since, due to limited diffusion rates of charges through a solid, their operation is temperature dependent to a much larger extent, and many operate well only under elevated temperatures. Due to the limited diffusion rates, solid state batteries have a low ratio of electrical energy generated versus their potential chemical energy.
Liquid state, thin layer batteries typically include a positive and a negative active insoluble material layers put together with a porous separator layer interposed therebetween, which separator layer is soaked with a liquid electrolyte solution. Such batteries, examples of which are described in U.S. Pat. No. 4,623,598 to Waki et al., and in Japanese Patent Document JP 61-55866 to Fuminobu et al., have to be sealed within a sheathing film to prevent liquid evaporation, and are therefore closed electrochemical cells. Being closed cells, these batteries tend to swell upon storage due to evolution of gases. This is a fatal problem in thin layer batteries having no mechanical support, because the pressure imposed by the accumulated gases leads to separation of the layers, making the battery inoperative. Means to overcome this problem include: (i) the use of a polymer increased-viscosity agent, such as hydroxyethylcellulose, which bonds the battery layers together and serves as a mechanical support; and (ii) the addition of mercury to prevent the formation of gases, especially hydrogen. However, the polymer is limited in its effectiveness and the mercury is environmental hazardous.
Another solution to the problem of the evolution of gases in liquid state batteries is proposed in U.S. Pat. No. 3,901,732 to Kis et al., in which a gas-permeable, liquid-impermeable polymeric material is used as a sheathing film to enclose the battery cell. The polymeric material allows venting of undesirable gases formed within the battery while preventing any liquid loss from the battery.
A more direct and efficient approach to avoid undesired gas accumulation in liquid state thin layer batteries is disclosed in U.S. Pat. Nos. 5,652,043 and 5,897,522 to Nitzan, the disclosures of which are incorporated herein by reference. U.S. Pat. Nos. 5,652,043 and 5,897,522 disclose liquid state batteries constructed as open cells for facilitated release of gases, while at the same time are constructed to avoid liquid evaporation and drying out of the batteries. Such a construction enables production of thin layer batteries devoid of casings, which batteries are simpler, thinner, more flexible and cheaper for mass production.
Reference is now made to FIG. 1, which illustrates a basic configuration of the flexible, thin layer, open electrochemical cell of U.S. Pat. Nos. 5,652,043 and 5,897,522, referred to hereinbelow as cell 10.
Cell 10 includes a layer of an insoluble negative pole 14, a layer of an insoluble positive pole 16 and a layer of a porous separator 12 interposed therebetween. The layer of the separator allows ionic transfer of electrolyte between the pole layers. It is noted that throughout the specification herein, the negative pole is where oxidation occurs, whereas the positive pole is where reduction occurs.
Separator 12 typically includes a porous insoluble substance, such as, but not limited to, filter paper, plastic membrane, cellulose membrane, cloth, non-woven material (e.g., cotton fibers) and the like, and is soaked with the aqueous electrolyte solution. The electrolyte solution typically includes a deliquescent (and hygroscopic) material for keeping open cell 10 wet at all times, an electroactive soluble material for obtaining the required ionic conductivity and a water-soluble polymer for obtaining the required viscosity for adhering pole layers 14 and 16 to separator 12.
By being hygroscopic, the deliquescent material maintains open cell 10 moisturized at all times. The level of moisture within the cell may vary depending on the deliquescent material selection, its concentration and ambient humidity and temperature. Suitable deliquescent materials include, but are not limited to, calcium chloride, calcium bromide, lithium chloride, zinc chloride, potassium biphosphate, potassium acetate and combinations thereof.
The electroactive soluble material is selected in accordance with the materials of which the negative and positive pole layers are made. Examples of suitable electroactive soluble materials include zinc chloride and zinc bromide for various primary cells, and potassium hydroxide and sulfuric acid for rechargeable cells.
The water-soluble polymer is employed as an adhesive agent to bond pole layers 14 and 16 to separator 12. Many types of polymers are suitable for this purpose, such as, for example, polyvinyl alcohol (PVA), poly(acryl amide), poly(acrylic acid), poly(vinyl pyrrolidone) (PVP), polyethylene oxide, agar, starch, hydroxyethylcellulose and combinations and copolymers thereof.
In all of the embodiments of U.S. Pat. Nos. 5,652,043 and 5,897,522, the porous separator layer is formed or manufactured as a separate layer from the negative and positive pole layers. This, in turn, is limiting in the sense that (i) it calls for at least three manufacturing steps; and (ii) the contact between the layers is typically less than optimal.